Problem: What is the least common multiple of 21 and 30? Another way to say this is: $\lcm(21, 30) = {?}$
Answer: The least common multiple is the smallest number that is a multiple of 21 and 30. We know that 21 x 30 (or 630) is a common multiple, but is it the least common multiple? Write out the multiples of 30 until we find a number divisible by 21. 30, 60, 90, 120, 150, 180, 210, So, 210 is the least common multiple of 21 and 30.